The present invention relates to resettable PTC devices made of inorganic-metal composite materials, and more particularly to a body of such composite material having a room temperature resistivity of less than 10 xcexa9xc2x7cm and a high temperature resistivity of at least 100 xcexa9xc2x7cm.
Positive Temperature Coefficient (PTC) materials exhibit a sharp increase in resistivity over a particular temperature range. As such, these materials have been used widely as resettable fuses for protecting circuits against overcurrent conditions.
Two types of PTC materials have been proposed in the past: ceramic-based PTCs and polymer-based PTCs. Ceramic PTCs made of, for example, barium titanate, have been used in heaters and in some circuit protection applications. Ceramic PTCs have not been widely adopted for circuit protection devices, however, since the room temperature resistivity of those materials is too high for use in circuits of consumer electronic products, for example.
In view of the problems associated with ceramic PTC materials, the industry has adopted polymer-based materials. Such polymer-based PTC materials include a matrix of polymer material in which conductive particles, such as carbon black, are uniformly dispersed to form a conductive network through the material. The resistivity of the polymer PTC is controlled by varying the content of conductive particles. The range of conductive particle content within which the polymer composite material exhibits PTC behavior is known as the percolation threshold range.
FIG. 1 is an operating curve for a typical polymeric PTC device. The PTC device will generate heat as current passes therethrough. The device will operate in region 1 as long as the amount of heat generated in the device can be dissipated to the ambient environment. In an overcurrent condition, the heat generated by the device exceeds the ability of the ambient environment to absorb that heat, and, consequently, the temperature of the device increases. When the temperature of the device reaches the melting point temperature of the polymer matrix, the polymer melts, expands and disrupts the conductive network of carbon black particles formed therein. Once the conductive network is disrupted, the resistivity of the polymeric material increases sharply as shown in FIG. 1, to thus allow only a very small amount of current to pass therethrough. Region 3 shown in FIG. 1 basically represents the resistivity of the polymeric composite material in the melted state. Once the overcurrent condition is terminated (e.g., by switching off the electronic device), the polymer recrystallizes and effectively reconstructs the conductive network of carbon black particles. The device then operates in region 1 of FIG. 1 until a subsequent overcurrent condition occurs.
While polymeric PTC devices have been widely adopted in industry, there are several problems associated with these devices.
First, while the magnitude of resistivity in region 1 of a polymeric PTC device can be adjusted by changing the amount of conductive particles added to the polymer matrix, the trip point temperature (TTP) is dependent solely upon the melting point of the polymer. Polyethylene is the material of choice in polymeric PTC devices, and melts at about 150xc2x0 C. Accordingly, all polymeric PTC devices employing polyethylene as the matrix material will trip when the device temperature reaches 150xc2x0 C.
Second, the breakdown voltage of polymeric PTC devices is relatively low (e.g., less than 100 V/mm), primarily due to the relatively low breakdown voltage of polymer materials such as polyethylene.
Third, there is a time lag between the occurrence of an overcurrent condition and the tripping of the polymeric PTC device. Specifically, the xe2x80x9ctrip timexe2x80x9d of a polymeric PTC device is on the order of 100 milliseconds. Consequently, some or all of the overcurrent could be transmitted to downstream electronic components within this time lag.
Fourth, polymeric PTC devices do not return to their initial resistivity value after tripping. Specifically, the first time a polymeric PTC device trips, and the polymer matrix melts as explained above, the initial conductive network of carbon black particles is disrupted. The carbon black particles do not assume the same network when the polymeric matrix cools to region 1 of FIG. 1 since the structure of the polymer matrix changes slightly. Consequently, the magnitude of resistivity in region 1 essentially doubles after the polymeric PTC device is tripped for the first time. Such an increase in region 1 resistivity is unacceptable, especially in devices where the initial resistivity of the polymeric PTC device plays an important role in the design of the electronic circuit.
Fifth, polymeric PTC devices require several hours, if not several days, to reset. Specifically, once the polymeric matrix melts as a result of an overcurrent condition, it could take several hours or days for the polymeric matrix to recrystallize and again become conductive (by restoration of the conductive network of carbon particles). This is unacceptable since an electronic device in which the polymeric PTC device is disposed cannot operate until the PTC device resets.
Sixth, the heat resistance of polymeric PTC devices is unacceptably low (i.e., less than 200xc2x0 C.). As explained above, the polymeric matrix, if formed of polyethylene, will melt at about 150xc2x0 C. to disrupt the conductive network of carbon black particles in the device. However, in certain severe overcurrent conditions, the PTC device itself can be heated above the melting point of the polymer and perhaps even above the decomposition temperature of the polymer itself. That is, a severe overcurrent condition can cause decomposition of the polymer matrix if the current flowing through the device generates excessive Joule heating. Decomposition of a polymeric material essentially forms carbon (which is electrically conductive) and essentially renders the device permanently inoperative. Accordingly, the PTC device is no longer resettable.
Finally, certain overcurrent conditions can cause shorting around the ends of the polymeric material (known as xe2x80x9ctrackingxe2x80x9d) and even through certain local regions of the polymeric material. These short circuit conditions create local areas of decomposition in the polymeric material, which in turn result in permanent conductive paths of carbon in the device. Such conductive paths are, of course, unacceptable, as the device will no longer exhibit a sharp increase in resistivity at the trip point temperature.
It would be desirable to develop a PTC material that does not suffer from the excessive resistivity problems of traditional ceramic PTC materials and also does not suffer from the numerous drawbacks associated with polymeric PTC materials.
While extensive research has been conducted in the area of polymeric PTC devices in an attempt to overcome some of the above problems, the industry, until recently, had not been able to provide a PTC material that overcomes all of the problems discussed above with respect to both traditional ceramic and polymeric PTC materials. There has been recent disclosure, however, of a PTC thermistor material including a ceramic matrix and conductive particles dispersed therein. Specifically, WO 98/11568 (EP0862191) discloses such a composite material device that purports to exhibit reliable PTC behavior. However, the device must make use of a semi-insulating matrix material in order to attain acceptably low room temperature resistivity. While insulating ceramic matrix materials (e.g., Al2O3) are disclosed, the room temperature resistivity of the devices employing these materials is unacceptably high (xcx9c1000 xcexa9xc2x7cm). Moreover, the use of semi-insulating matrix materials often results in unacceptably low high temperature resistivities (above the trip point temperature of the device), and the cost of such semi-insulating materials tends to be prohibitive. Accordingly, WO ""568 does not disclose a device that simultaneously can achieve low (e.g.  less than 10 xcexa9xc2x7cm) room temperature resistivity and acceptable high temperature resistivity, while being made of a relatively inexpensive matrix material.
It is an object of the present invention to provide a PTC material that overcomes all of the above-discussed drawbacks associated with conventional ceramic and polymeric PTC materials.
Specifically, it is an object of the present invention to provide an inorganic-metal composite body that exhibits reliable PTC behavior over a broad range of selectable trip point temperatures. The composite body of the present invention can be made from relatively inexpensive inorganic materials, such as insulating ceramic materials, while still exhibiting relatively low room temperature resistivity (xe2x89xa610 xcexa9xc2x7cm) and a resistivity ratio (high temperature resistivity/room temperature resistivity) of at least 10.
In accordance with one object of the present invention, an inorganic-metal composite body is provided that exhibits PTC behavior at a trip point temperature ranging from 40xc2x0 C.-300xc2x0 C., and comprises an electrically insulating inorganic matrix having a room temperature resistivity of at least 1xc3x97106 xcexa9xc2x7cm, and electrically conductive particles uniformly dispersed in the matrix to form a three-dimensional conductive network extending from a first surface of said body to an opposed second surface thereof. The composite body has a room temperature resistivity of no more than 10 xcexa9xc2x7cm and a high temperature resistivity, above the trip point temperature, of at least 100 xcexa9xc2x7cm, preferably at least 1000 xcexa9xc2x7cm, and more preferably at least 10,000 xcexa9xc2x7cm.
The force that drives the PTC behavior in the composite body of the present invention lies in the ability of the electrically conductive particles to shrink at least 0.5% by volume at or above the melting point thereof. When excessive current passes through the body, the heat generated in the body causes the conductive particles to melt, shrink, and thus disrupt the conductive network passing through the body. This is the same basic manner in which the materials of WO ""568 purport to function as PTC devices.
During the course of the inventor""s research, it was discovered that the inherent defects of the materials disclosed in WO ""568 could be overcome by focusing on the specific composition of the electrically conductive particles. Accordingly, another object of the present invention is to provide the above-described inorganic-metal composite body, wherein the electrically conductive particles consists essentially of Bi in an amount of at least 50 wt %, and at least one additional metal element selected from the group consisting of Sn, Pb, Cd, Sb and Ga. If the amount of Bi is less than 50 wt %, then the electrically conductive particles do not shrink to a sufficient extent so as to allow reliable PTC behavior in the composite body. Binary alloys made up of Bi and one of these other metals can be used, as can ternary alloys such as Bixe2x80x94Snxe2x80x94Ga, Bixe2x80x94Snxe2x80x94Pb and Bixe2x80x94Snxe2x80x94Cd.
During the course of the inventor""s research, it was also discovered that the inherent defects of the materials disclosed in WO ""568 could be overcome by focusing on the particle sizes and particle size distributions used in formulating the electrically insulating inorganic matrix and electrically conductive particles. That is, the inventor discovered that a specific relationship should exist between the size of the inorganic particles used to make the matrix and the size of the electrically conductive particles in order to provide sufficient and uniform spacing between the electrically conductive particles in the final sintered body. Complete disclosure of this discovery is outlined in applicant""s copending U.S. application Ser. No. 09/324,263, filed Jun. 2, 1999, the entirety of which is incorporated herein by reference.
The inventor also discovered that the particle size distribution of the electrically conductive particles is important in providing the composite body with acceptably low room temperature resistivity (i.e., less than 10 xcexa9xc2x7cm) within the percolation range of the material. Accordingly, it is another object of the present invention to provide the above-described composite body with electrically conductive particles having an average particle size (xcfx86ave) ranging from 5 microns to 50 microns and a 3"sgr" particle size distribution ranging from 0.5 xcfx86ave to 2.0 xcfx86ave. It is also preferred that no more than 5 vol % of the electrically conductive particles in the composite body be smaller than 5 microns.
While researching the composite body of the present invention, the inventor also discovered that traditional electrode termination techniques could not be used. Specifically, it was discovered that the bond between conventional (e.g., Ni, Ag, Cu) electrodes formed on the outer surface of the composite body and the constituents of the composite body would deteriorate each time the conductive particles in the composite body melted. In addition, the alloy particles in the composite body would migrate toward the conventional electrode materials and form an alloy, thus leaving a depleted area within the composite body that increased the resistivity of the overall device. Accordingly, another object of the present invention is to provide an inorganic-metal composite body that exhibits reliable PTC behavior, while enabling the use of conventional electrode termination materials, such as Ni, Ag and Cu. In accordance with this object of the invention, an inorganic-metal composite body is provided that preferably includes the composite body described above, an intermediate layer and an outer electrode layer. The intermediate layer includes inorganic particles, preferably the same as the composite body, and an electrically conductive network formed therethrough. The electrically conductive network is defined by a metal or alloy that (i) has a higher melting point temperature than that of the conductive particles in the composite body, and (ii) will not form a eutectic alloy with the conductive particles in the composite body either during manufacture or use of the device. Use of such an intermediate layer enables the use of conventional electrodes to terminate the opposite ends of the composite body according to the present invention.
In addition to the above, the inventor discovered that use of electrically conductive particles having relatively low melting point temperatures presents difficulty when attempting to manufacture the composite body of the present invention using traditional ceramic processing techniques. Specifically, electrically insulating materials such as alumina, mullite, and the like, are typically fired at 1200-1500xc2x0 C. However, the vaporization temperature of most bismuth-based alloys is but a fraction of that sintering temperature. Accordingly, traditional firing techniques must be modified to prevent vaporization of the electrically conductive particles during formation of the fired inorganic-metal composite body.
Accordingly, it is yet another object of the present invention to provide a method of making the above-described composite body, wherein an additive is added to the batch material that includes the electrically insulating inorganic material and the electrically conductive particles, to act as a vaporization suppressing aid during sintering of the composite body. The vaporization suppressing aid is preferably a glass-based sintering aid having a glass transition temperature that is lower than the vaporization temperature of the electrically conductive particles included in the batch material. The additive melts during the sintering operation at a temperature below the vaporization temperature of the electrically conductive particles, and forms an envelope around the electrically conductive particles that effectively prevents the vaporized material from escaping the composite body. Use of such a vaporization suppressing aid preserves the amount of electrically conductive material in the final sintered composite body.